Diabetes mellitus is a chronic disease that causes systemic metabolic disorders such as hyperglycemia, glycosuria, disintegration of body proteins, ketosis, or acidosis. In general, diabetes is roughly divided into type 1 and type 2 diabetes, wherein β cells in the pancreas are for some reason disrupted to deplete insulin which regulates blood glucose levels in the type 1; whereas insulin is present in the blood but does not function properly or the amount of insulin secreted from pancreatic β cells decreases, resulting in impaired regulation of blood glucose level in the type 2.
Recently, as a hormone that regulates the blood glucose level, incretins which are one of the gastrointestinal hormones have drawn much attention. The incretin is a collective term for hormones that are secreted from the gastrointestinal tract in association with food ingestion and that acts on pancreatic β cells to promote insulin secretion. There are two known incretins, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The secreted incretin binds to its receptor on the surface of pancreas β cells to promote the insulin secretion and suppress glucagon secretion, thereby exhibiting an effect of lowering the blood glucose level. The effect of promoting the insulin secretion by incretins depends on glucose concentration in the blood and is brought out only when glucose is present at a certain concentration or higher. That is, there is a less risk of occurrence of hypoglycemia which is a concern in direct insulin administration which is a conventional treatment method; and after-meal hyperglycemia can be expected to be safely corrected. As a matter of fact, it has been revealed that continuous administration of GLP-1 which is one of the incretins to patients with type 2 diabetes via the veins promotes the insulin secretion and thereby blood glucose control is significantly improved, demonstrating usefulness of GLP-1 supplemental therapy.
Meanwhile, GLP-1 is rapidly degraded by dipeptidyl peptidase-4 (EC3.4.14.5, hereinafter referred to simply as DPP-4) which is extensively present in the living body. In view of this, as a method of treating diabetes, new drug agents have been developed, which new drug agents inhibit the activity of DPP-4 to keep and enhance an effect of endogenous GLP-1. It is to be noted that DPP-4 is a serine protease that is expressed on the plasma membrane as CD26 of immune cells such as T cells and is also present as a soluble protein in the blood to inactivate GLP-1. DPP-4 specifically acts on a biologically active peptide with Pro or Ala at the second position from the N-terminal to allow dipeptide to be freed from the N-terminal.
On the basis of such an effect of DPP-4, a DPP-4 inhibitor comprising a peptide having 3 to 12 amino acids is available, wherein Pro is arranged at the second position from the N-terminal (Patent Literature 1). The DPP-4 inhibitor is a peptide that is contained in a water-soluble fraction obtained by suspending cheese in an aqueous solvent and then removing insoluble substances; and when orally taken, a food or drink in which such a peptide is contained can lower blood glucose levels in the living body. The DPP-4 inhibitor described in Patent Literature 1 is obtained from a natural product and is thus said to be low in toxicity and high in safety.
Further, also available is a peptide that is a milk protein hydrolysate, stimulates GLP-1 secretion, and has a DPP-4 inhibitory effect (Patent Literature 2). It is described that the peptide disclosed in Patent Literature 2 is preferably one with 2 to 8 amino acids in length that contains Pro as the second N-terminal residue. In the examples, as a certain milk protein hydrolysate, a peptide composition of 500 to 2000 Da was evaluated for the DPP-4 inhibitory effect thereof, yet the composition and peptide sequence thereof are unknown.
Further, also available is a DPP-4 inhibitor that contains a preparation derived from a material for eating or drinking as an active component (Patent Literature 3). It is one that contains, as an active component, a preparation derived from a material for eating and drinking, the preparation exhibiting a DPP-4 inhibition rate of 60% or more at a solid concentration of 3.5 mg/ml or less. In the examples in Patent Literature 3, peptides derived from mung beans, soybeans, collagen, seaweed, green tea, walnuts, tian cha, pomegranate, grape seeds, and the like are evaluated for the DPP-4 inhibition rate thereof.
Further, also available as a DPP-4 inhibitor is a peptide derived from collagen or gelatin, which peptide is represented by Gly-X-Y-(Gly-Z-W)n (SEQ ID NO: 15) (wherein n is an integer of 0 to 4; X is Pro or Leu; Y, Z, and W are the same or different and represent each independently any amino acid residue (with the proviso that Gly is excluded) (Patent Literature 4). In the examples, a commercially available collagen peptide is fractionated by high performance liquid chromatography and the sequence of peptide in a fraction that has an excellent DPP-4 inhibitory activity is specified.
Meanwhile, as an effect of a DPP-4 inhibitor, suppression of T lymphocyte proliferation has been known (Non Patent Literature 1). Non Patent Literature 1 points out that the DPP-4 inhibitor can alleviate autoimmune diseases such as autoimmune spondylitis, multiple sclerosis, arthritis, or rheumatism. In addition, because DPP-4 is involved in acceleration of HIV-1 infection through CD4 positive T cells, it has also been reported that use of a DPP-4 inhibitor in combination with other drug agents is expected to protect against HIV-1 infection (Non Patent Literature 2). Further, because DPP-4 is expressed by skin cells, it has also been reported that a DPP-4 inhibitor may have an influence on proliferation, differentiation, or cytokine production of sebaceous gland cells or epidermal cells (Non Patent Literature 3). It is implicated that T lymphocyte proliferation stimulated by acne bacteria is suppressed by the DPP-4 inhibitor, which causes suppression of fibroblasts' TGF-β expression, suppression of fibroblast proliferation, suppression of fibroblasts' matrix production; and, as a result, acne, psoriasis, and keloid can be treated by the DPP-4 inhibitor.